# Is voltage and current in phase in a transmission line wih a lossless medium?

In electromagnetics, E and H is in phase in a transmission line wih a lossless medium.

So V and I is in phase.

But in power transmission line, usually I is lagging with V.

Since we mainly use inductive loads, although we are using various kinds of reactive power compensation devices, the current lags behind the voltage by the phase difference in the power transmission line.

Then, E and H must be out of phase.

Then, intrinsic constant of E and H ??

How should I understand ??

• in power transmission line, usually I is lagging with V <-- where is the justification for this? I mean it can be and, this is usually due to the transmission line being run at 50 or 60 Hz, but, what do you understand about characteristic impedance at low frequencies? Have you done the math? Commented Feb 25, 2023 at 8:13
• I guess this would depend on the load type as shown in the chart. Commented Feb 25, 2023 at 8:42
• @user253751 If it depends on the Load, is it possible that E and H is out of phase in inductive load ?? Then, intrinsic constant of E and H ??
– user331400
Commented Feb 25, 2023 at 8:50
• @BriL I'm a moderator. Andy is correct in suggesting that changing a question so that a correct answer is made incorrect by the alteration is something that should be avoided if possible. Commented Feb 25, 2023 at 11:27
• @Andyaka Rolling back one's own questions or answers when someone hacks them about excessively is usually reasonable. Rolling back someone else's question when they edit it themselves is probably "risky". [Consider: You'd be extremely unhappy if someone did that to you, regardless of why.] The reason that you did so is understood, and the OP would benefit from your suggestions. But, ... . Commented Feb 25, 2023 at 11:30

The difference is due to comparing:

• the behaviour of a transmission line when the signal has not yet had time to reach the end (or the line is modelled as infinite)

A lossless transmission line appears as a simple resistor "absorbing" energy into the E & H fields passing down the line. These is no return flow, so the fields are in sync.

If the line is finite, but terminated at its characteristic impedance, still no energy returns and the fields stay in sync.

Once you look at reflections, or the impact of reactive components at the end, you have currents / energy flowing both ways, and you then get phase shifts.

• I really appreciate for your guidance. Then, the phase difference between E and H is exactly the same as the phase difference between V and I. Is it right? And poynting vector, active_reactive power is also related. right?
– user331400
Commented Feb 25, 2023 at 13:06
• Yes (possibly give or take a minus sign, I can't remember at present). Commented Feb 25, 2023 at 13:20
• Thank you very much for your comment! Can I find the textbook or article or website dealing with this content (the phase difference between E and H, poynting vector, active_reactive power) ? because general textbooks do not contain the relationship between the phase difference between E and H of the Poynting vector and active reactive power....
– user331400
Commented Feb 25, 2023 at 13:30
• You may find this helpful aip.scitation.org/doi/10.1063/5.0101339 and this youtube video youtube.com/watch?v=bHIhgxav9LY (though there are grey areas on some of the latter's analysis) Commented Feb 25, 2023 at 14:24
• Thank you very much for your comment! I've already seen that youtube video and it doesn't show or deal with the phase difference between E and H. But I will take a good look at the site you mentioned and research it. Thank you so much!!
– user331400
Commented Feb 25, 2023 at 14:33